382 CHEMICAL ENGINEERING
evaporators to 75 or 80 per cent. In order to prevent contamination of the extracts and corrosion, it is necessary to make these evaporators of copper and bronze entirely. The weak liquors will foam considerably and scale will form on the tubes especially if hard water is used for the leaching process. The vertical-tube evaporator is commonly used for tanning extracts, but the rapid circulation and film types are excellent machines for this purpose.
Cascara, Coffee, Licorice, Nicotine, Tea.—The original density of these extracts depends entirely on the manufacturing process, and no average can be given. The liquors are usually concentrated to from 40 to 50 per cent solids, and the capacity is about 2 gal. per square foot with a steam pressure of 5 lb., and a vacuum of from 27 to 28 in. Single effects of the horizontal-tube or rapid-circulation type are standard equipment; sometimes a plain coil pan with or without steam jacket is used. Heating surface must be of copper and the shells can be of either copper or enameled steel.
Tartaric Acid.—The usual concentration of the filtered solution is from 30 to 70 per cent of solids, and the work is done in single-effect evaporators made entirely of hard lead with extra-heavy lead coils. The all-lead construction may be replaced by a cast-iron shell with suitable lead lining, and the tubular heating surface with vertical tubes and lead flueplates can be used instead of the lead coils. It is very important that the construction is such that tubes can readily be cleaned from the hard scale produced by the calcium sulphate in the solution. With a steam pressure of from 30 to 50 lb. in the coils, and a vacuum of 27 in., the capacity is from IK to 2 gal. per square foot.
Lactic Acid.—The filtered solution is usually concentrated from 8 to 50 per cent in a single-effect evaporator of the vertical-tube or rapid-circulation type. All parts of the evaporator coming in contact with the liquor or vapor must be of copper. The solution contains calcium sulphate, and the deposit of hard scale must be removed very frequently from the tubes.
PyroEgneous Acid.—One cord of wood will give from 250 to 300 gal. of crude acid. A mixture of acetic acid, alcohol and water is distilled from the crude acid in double- or triple-effect evaporators of the vertical-tube or rapid-circulation type. The total evaporation is from 90 to 95 per cent, and the capacity is from 2 to 3 gal. per square foot, with a steam pressure of 5 lb. and a vacuum of 27 in. A surface condenser is attached for the recovery of the watery acid and alcohol. Evaporators must be built entirely of copper. Heating surface is frequently covered by a heavy coating of tar and charcoal dust, which has to be removed by mechanical cleaning or may be dissolved by the crude acid.
Acetate of Lime.—One cord of wood will give from 180 to 220 gal. of solution which is usually concentrated from about 6 to 28 per cent and sometimes as high as 30 per cent solids. With a steam pressure of 5 lb., and a vacuum of 27 in., the capacity is from 1% to 2 gal. per square foot in double or triple effects of the vertical-tube or rapid-circulation type. Evaporator shells are made of cast iron with copper or steel tubes.
For smaller installations, the concentration of the crude pyroligneous acid and acetate of lime is done in a double-effect evaporator, with the acid in the second effect.
Acetate of Soda.—The usual concentration is from 5 to 33 per cent and from 33 per cent to the crystallizing point, and the salts are recovered in standard salt